This application is related to the field of digital communications and more specifically to communications systems that require waveforms which occupy a fixed bandwidth channel.
For digital transmission over band limited channels, the demand for bandwidth efficient signaling schemes has increased. A system often used for band limited channels is multi-bit per symbol phase shift keying (M-ary PSK) which has the drawback that for M equal to 2 or 4, the signal possesses a wide band because of discontinuous phase. Thus, radio frequency filtering has to be performed before transmission causing decreased receiver sensitivity. Other systems such as minimum shift keying (MSK) and fast frequency shift keying (FFSK) possess an error probability performance similar to 2- or 4-ary PSK but with a narrower spectrum for large frequencies. Choosing an M larger than 4 (e.g., M=8 or M=16) in the MPSK system makes the main lobe of the spectrum narrower, but increases the system's sensitivity to noise.
Continuous phase modulation (CPM) signals, as discussed in Digital Phase Modulation by Anderson, J., Aulin, T. and Sundberg, C. E., the entirety of which is herein incorporated by reference, have many advantages over phase shift keying (PSK) signals. PSK signals must be filtered and transmitted through linear amplifiers. After filtering, PSK signals have an amplitude variance that must be accounted for to prevent signal distortion and transmit power amplifier over-current. CPM signals do not possess this problem and may be transmitted at the maximum power level allowed by a radio power amplifier. To transmit at the same amount of power of the CPM signal, a PSK signal would require a power increase at the amplifier generally on the order of 4-5 dB. On the contrary, a non-constant envelope (NCE) CPM signal would require a power amplifier that only provided 1.2 dB more power. Any loss in bit error rate (BER) of the NCE-CPM signal with respect to the PSK signal may be compensated by the trade-off in power amplifier transmitted power.
Further improvements may be realized with NCE-CPM signals. For example, these signals may possess multiple modulation indices, h, which relate the size of the baseband pulse of a signal to the phase variation. A multi-h signal has a natural trellis structure that may be used to improve the modem BER performance without additional, redundant parity bits. Multi-h codes are phase codes in which the modulation index varies in a cyclic pattern from interval to interval. When this feature is added to an existing trellis encoder, a concatenated code results, in which trellis paths remain apart longer and minimum distance improves. This changes the modulation index of each symbol thereby delaying the point at which phase trajectories with different starting symbols remerge. This increases the minimum Euclidean distance (constraint length) and reduces the probability of symbol error. The cost for obtaining better detection efficiency through the use of a multi-h scheme is an increase in receiver complexity as compared to the single-h case. The optimum decoder for a trellis code in Gaussian noise is the Viterbi algorithm which traverses every path in the trellis structure to find the optimum path. An NCE-CPM waveform has both a reduced transmit signal spectra in comparison to a CPM waveform and the inherent CPM multi-h trellis structure which makes the NCE-CPM signal a better overall waveform for band limited channel communications. Furthermore, the NCE-CPM signal may be demodulated by a standard CPM maximum likelihood demodulator without any loss of performance. This allows an NCE-CPM signal a degree of interoperability with existing CPM demodulation capable receivers.
A performance summary of PSK, CPM and NCE-CPM waveforms is shown below in Table 1:
Power (Peak toModulationAverage)BandwidthBER PerformancePSK+4 dB to 5 dBBase10-5 at 9.5 dBCPMunity−25%10-5 at 8.5 dBNCE-CPM+1.2 dBSame bandwidth as10-5 at 12 dBPSK
It is therefore an object of the disclosure to present a method for improving the bandwidth efficiency of a CPM signal comprised of plural symbols. The method includes the steps of coding the data stream, modulating the data stream with a constant envelope CPM waveform and converting the constant envelope CPM waveform into an NCE-CPM waveform.
It is another object of the disclosure to present a method for improving the bandwidth efficiency of a constant envelope CPM waveform signal encoded with data symbols comprising the steps of varying a complex amplitude of a signal between successive constellation points prior to transmission and controlling the complex amplitude of the signal to follow a path between constellation points.
It is a further object of the disclosure to present a method of transmitting data as an NCE-CPM signal comprised of a plurality of symbols in a constellation. The method includes the steps of coding the plurality of symbols and transecting each symbol by direct path during modulation of the coded plurality of symbols.
It is still another object of the disclosure to present a method for modulating an input data stream comprised of a plurality of symbols represented as constellation positions in a complex plane. The method includes the steps of generating a CPM waveform modulated with the plurality of symbols and modifying the complex amplitude between the successive constellation positions to traverse the complex plane in a straight path from constellation position to constellation position.
It is an additional object of the disclosure to present a system for improving the bandwidth efficiency of a CPM waveform communication system. The system includes a transmitter for transmitting data as CPM symbols, a receiver with a constant envelope CPM demodulator, and a conversion means for converting a constant envelope CPM waveform to an NCE-CPM waveform prior to transmission such that the complex amplitude of the signal between successive constellation points is less than the complex amplitude at each of the constellation points.
It is an object of the disclosure to present a method for transmitting an input data stream as phase locations in a complex plane in a CPM waveform wherein the CPM waveform modulated with the input data stream transects the complex plan between successive phase locations in a substantially straight path.
It is also an object of the disclosure to present a method for communicating data wherein the data is transmitted as a plurality of symbols with a complex plane as a CPM waveform wherein the magnitude of the waveform in the complex plane varies between symbols.
These and many other objects and advantages of the present disclosure will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.